Microplastics (MPs) are transported from terrestrial environments to the ocean through rivers. However, the timescale of this transport and associated degradation process remains poorly understood. To clarify the transport timescale and degradation of MPs from land to the ocean, we collected MPs using fixed sampling devices in urban rivers in Fukuoka, Japan. We then identified the shape, size, and material of the collected MPs using optical microscopy and FT-IR analysis. The age of 39 polyethylene (PE) particles was then estimated using the carbonyl index (CI). The estimated ages ranged widely from less than one year to over ten years, exhibiting a broad distribution without pronounced peaks at specific ages. This age distribution differs significantly from that observed in the ocean surface layer, where relatively young MPs predominate.

To investigate the mechanism underlying the observed riverine MPs age distribution, I developed a simplified advection model that accounts for retention by banks, resuspension due to rainfall, and the initial age of MPs before entering rivers. Model results indicate that the observed broad age distribution can be reproduced only when long-term retention at sandbanks and substantial degradation of MPs occur before entering rivers. These findings suggest that significant generation and degradation of MPs occur in terrestrial and river environments before their transport to the ocean.

microplastic,age,river,advection model